This invention relates to digital telecommunications and more particularly to digital telecommunications in the megabit/second data rate range using a standard telephone twisted-pair cable a few thousand feet in length. The invention is primarily for use in local loop digital subscriber telephone systems capable of supporting voice, data and image transmission. However, apparatus according to the invention may also be used in connection with data acquisition and digital control tasks.
The transmission of information in digital form at frequencies exceeding one megabit/second has in the past generally been accomplished by means of expensive cabling systems, such as coaxial cable, shielded twisted-pair cable, for fiberoptic cable. In the past, such systems have generally required sophisticated data recovery techniques including analog equalization and phase synchronization.
While the use of such known techniques can be justified for communication between computer equipment where expense is not a primary factor, there is nevertheless a need to provide a relatively inexpensive and yet reliable system optimized for use with inexpensive, often previously installed, standard twisted-pair wiring. A transceiver for this purpose must not only meet economic criteria but also satisfy stringent bit error rate standards, radiation standards, and cross-talk standards.
There is a growing need to provide for the interface of telephones with various types of digital communication equipment, including personal computers and the like, to previously installed twisted-pair wiring. One possible technique for implementing shared voice and data communication is through the use of packet switching wherein packets contain voice, data or control and signaling information. To be effective, packet switched communications must be at a rate high enough to allow transmission of voice signals with minimal delays in order to permit conversational communication. In a PBX environment where it is desired to minimize the number of wires, it is preferable to provide power over the same physical wires which carry data. This imposes the requirement that digital data be in an ac form free of dc offsets. The transmitters and receivers can thus be coupled to the transmission medium by means of pulse transformers, and data and coding schemes such as alternate mark inversion (AMI) must be employed. While AMI encoding schemes are appropriate for transformer coupled lines, AMI schemes are not self-clocking so synchronization is easily lost. The conventional solution is to provide modified AMI schemes to take into account arbitrarily long sequences of zeros which result in arbitrarily long periods between pulses. The modified schemes involve insertion of intentional bipolar violations in the data stream. Detection schemes for such systems are generally complex and expensive.
High frequency transmission system operating over distances exceeding a few hundred feet generally adopt a self-encoding scheme and employ phase locked loop synchronization techniques with continuous frequency and/or phase tracking. Phase locked loop synchronization techniques require a finite acquisition time at the beginning of each frame, so bandwidth constraints and delays seriously hinder use in a high-speed environment with frequent synchronization. Moreover, four wire systems are frequently required to achieve full duplex capability, since in prior art schemes, line turnaround is slowed by synchronization delays.
Another problem noted, particularly with the standard twisted-pair cabling over a length of several hundred feet, is the low pass filter effect caused by the lines themselves. The medium dependent low pass filter characteristic causes distortion which increases the problem of intersymbol interference when data rates approach the bandwidth of the medium. In the past, the low pass filter characteristics have been compensated for at the receiving end of a channel by means of analog equalization to achieve acceptable bit error rates. Analog equalization does not lend itself to digital integration, thus reducing any advantages gained by large scale integration of other circuits. Still further, intersymbol interference imposes conflicting requirements on the level detection threshold used to distinguish between logic values ONE and ZERO. For example, the voltage threshold used to separate pulses from the absence of pulses is preferably as low as possible in order to compensate for effect of intersymbol interference. Transmission of a pulse followed by the absence of pulses may be detected incorrectly at the receiving end as consecutive pulses if the threshold voltage used to separate the pulses from the absence of pulses is set at a low value. This false reading is due to the relatively long discharge time of a transmission medium acting as a low pass filter. Straightforward detection schemes are further complicated because a DC free coding scheme such as AMI is non-self clocking. A clock signal is frequently embedded in an AMI code for synchronization purposes. Unfortunately a clock signal may create a DC artifact.
These and other problems have been addressed and overcome in the development of a high-speed digital transceiver suitable for voice and data interchange in a PBX environment.